Directional aerial



Feb. 13, 1934. w. VAN B; ROBERTS DIRECTIONAL AERIAL Filed March l, 1929@we/woz -ER VAN B. ROBERTS Patented Feb. 13, 1934 LM'LZI DmEc'noNALAERIAL Walter van B. Roberts, Princeton, N. J., assigrior to RadioCorporation of America, a corporation of Delaware Application March 1,1929.- Serial No. 343,710

2 Claims.

In radio direction finders known heretofore of the type which use arotatable loop aerial much diiculty has been encountered in obtaining asystem in which at zero response in the receiver energized thereby, theloop aerial will be pointing directly toward the source of energy.

This inaccuracy in true pointing position is due in part to distortedwave front and in part to characteristics inherent in the apparatus usedsuch as, for instance, l-out of phase currents caused by the verticalaerial or electrostatic effeet of the signal voltage on the tuned loopwhen the loop is unsymmetrical with respect to ground. This eiect ispresent whether the wave front is distorted or not; 2-currents due tosignal voltage across a loop of some axial length caused by a distortedwave front having horizontal component of the electric field. Thisvoltage is in phase with the electrical eld and the amount thereofchanges as the loop is rotated, and the current set up thereby is eitherin phase or 180 out of phase with respect to the normal loop eiectthereby distorting the normal loop characteristic curve. This eiect ispresent whether the loop is symmetrical or not, and results in shiftingthe point of zero response so that the loop does not point directlytoward the source of energy when signal response is minimum. Aerialsystems of the above type are subject to various other inherent defectscaused either by unsymmetrical apparatus, or thereby when eected byunsymmetrical signal waves, however, those pointed out above will servefor purposes of illustration.

Heretofore various means have been provided to compensate the variouserrors above referred to by adding current in the proper phase to moreor less compensate the error causing currents introduced in each of theabove mentioned ways. Such means known heretofore has necessitated theuse of considerable additional apparatus.

The primary object of this invention is to provide a new and simplemethod of correcting the null points of a loop shifted from truepointing position by the effect of a component of the electric eld alongthe axial length of the loop.

- Another object of this invention is to provide a loop in combinationwith simple means for compensating the error therein which once adjustedwill remain permanently adjusted for all wave lengths received on theloop.

A better understanding of the invention and the operation thereof willVbe had from the following detailed description of the inventionandtherefrom when read in connection with the annexed drawing throughoutwhich like references indicate like parts and in which Figure 1illustrates how the horizontal component of the electric field acts on atuned coil having appreciable length;

(Cl. Z-33) Figure 2 shows a method of winding an inductance coil toovercome the difficulties encountered with apparatus as illustrated inFigure 1;

Figure 3 shows novel correcting apparatus arranged to correct the errorin the coil of Figure 1 in accordance with the present invention; whileFigure 4 shows a receiving circuit connected with an aerial system whichincludes my novel method of reducingerror applied to a balanced frameaerial ,and a vertical aerial coupled to said frame aerial to obtain inthe receiver a cardioid characteristic curve.

Referring to the drawing and in particular to Figure 1 which shows asolenoidal coil or loop 1 tuned by means of a variable capacity 2. Whenthe axis X of the coil coincides with the direction of propagation ofthe signals we would normally have no current produced in the coil. But,if the wave front is distorted as indicated by the heavy line W F ofFigure 1 in addition to the expected vertical electric field which isperpendicular to the axis of the coil there would exist an electriccomponent along the flrectionof signal propagation, and then when theaxis of the coil coincides with the direction of signals a considerablecurrent will be produced by this horizontal electric component. This is,of course, due to the fact that the incoming wave which tends to varythe potential of all objects it meets, hits one end of the coil rst andthus causes a diierence of potential between the ends of the coil. Thisdiiference in potential in turn sets up currents. As a result such acoil generally does not give .a true indication of direction at itsposition of minimum signal response. The object of the present inventionis to provide a simple means for compensating and eliminating the errorintroduced in the above manner.

The difficulty may be obviated by using two coils wound in oppositedirections with their axis parallel as shown in Figure 2. The abovementioned trouble does not exist in this case because when an incomingelectric field is in the direction of the axis of the coil itself itstrikes both ends of the tuned circuit at the same instant, thusmaintaining the potentials at the two ends equal and hence producing nocurrent even though the eld of the approaching wave is distorted. Figure2 is only one of a number of symmetrical arrangements which may be usedto avoid the trouble.

It is possible however, to cure the trouble without going to the extracomplication of making the structure symmetrical. A modication showingan arrangement for eliminating such errors in accordance with thepresent invention is illustrated in Figure 3. In this modification twoconductors 3 and 4 from the right and left sides respectively of coil 1are run out to receive compensating differences of potential. The lengthof the conductors are determined-by trial and error and'once determinedfor a certain Vcoil need not be again altered. It will beV seen thattheY incoming wave strikes the conductor 4 extending thus offsets thefact that the wave strikes `the right hand end of the coil first.Likewise a wave approaching from the left 'of Figure 3V will strike theconductor 3 first and thereby compensate the effect which wouldotherwise be present in the coil due to the fact that the wave strikesthe left hand end thereof first. v

My invention is preferably used in connection withY other means known inthe art for convenientoperation of direction finders including auxiliarymeans fordetermining the sense as well as direction of the source ofradiant energy.

In Figure which shows the at present preferredembodiment of my inventionthe loop antenna 1 is provided with crossed electric pickup conductors 3and l provided for the purpose of eliminating error normally caused bythe current due to horizontal components of an electric field. Thelength of these pickup conductors is ascertained by trial and error fora particular loop and operate in the same manner in which the conductors3 and 4 of Figure 3 operate. vSense of direction is obtained by means ofa'vertical aerial 5 tuned by a variable capacity 6, coupled through aseries inductance '7 to an inductance 8 in series with the loop circuit1, 2. By tuning the aerial 5 and adjusting the coupling rbetween coils'l and 8 current ldue to the vertical aerial eiect in Y1 can beintroducedin the loop circuit in proper Yphase and amplitude to combinewith the current therein to obtain a cardioid characteristic curve inthe receivers. Energy from the aerial system is fed to the inputelements'of a receiver R by means o1 a tuned inductance 9 variablycoupled to an inductance 10 in series with the loop 1. To prevent thevertical component of the electric field of the wave from setting upcirculating currents in the tuned circuit the entire loop circuitincluding the coupling inductances is symmetrically balanced. This isattained by providing an inductance 11 electrically equivalent to theinductance 8 but in opposition thereto in series with the leads 12 and13 connecting the inductances 10 and 8 in series with the loop l. Tofurther insure the complete balance of the apparatus the electricalmidpoint of the inductance 10 is connected to ground through a connector14.

For the sake of a more quantitative description of the various currentswhich may be produced in theV tuned circuit 1, 2 of Figure 4 before thecompensating electric pickup adjustments have been made and thearrangement `has been properly balanced with respect to ground, I willtabulate them as follows:

1. The normal loop eect This is a voltage proportional to the rate ofV2. Ejectrof horizontal component of electric field This is proportionalto the electriceld andY to the cosine of the angle between the loop axisand the signals and produces a current 90 out of phase with the voltagebecause the voltage is applied across the tuned circuit rather than inseriesrin it'. Thus, the current is in time phase from the left end ofthe coil to the right rst and with the normal loop effect current and asa result makes the zero position of the loop fail A to coincide with.the direction of signals.

` 3, Effect of vertical electric field If the loop is not grounded atits electric cen- -ter the vertical eld will produce current inexactlythe same fashion as discussed under item 2. However, since thevertical field will act the same *.way, whatever direction the signalsare travelling the current produced will be constant and its phase'willbe the same as the phase of the currents in items 1 andv2.

4. The eect of the tufted 'vertical aerial on the loop If the auxiliaryopen antenna is tuned to the signal the current in its coil will be inphase with the electric iield and hence the magnetic eld of its couplingcoil 5 will Vbe inthe same phase as the magnetic eld of the incomingsignals. Thus, the current'V in the loop circuit produced by thecoupling inductance in the open antenna Y 5 will be in phase with thenormal loop current and by adjusting the coupling to such a value thatthe two currents aremequal in amplitude a cardioid diagram is obtained.Y

If this cardioid diagram is to be a perfect one, that isLif the systemis to have a true pointing position it is, of course, necessary that thetwo adjustments for wiping out currents produced by the electricxeld,`items 2 and 3 above, should have been made, namely, the grounding of theloopvcircuit at` such a point that no circulating currentis produced bythe vertical electric field, and Vapplying `the compensating electricpickups as disclosedin this invention to insure that any `horizontalcomponent lof the electric field will produce no circulatingcurrent inthe tuned circuit. 'if Y After the Vaerial has been installed' andthelength of the conductors ascertained by trial using signals from a knownsource, the operation of 4this vsystem is the same as the operation ofdirection finders of this type known heretofore. Having fully describedmy'invention and the operation thereof, I claim: 'f

ll. In a directional aerial system the'combination of a loop `aerial ofappreciable axial length,

and apair of crossed openended linear conductors parallel with the axisof said loop, opposite ends of. said conductors beingfconnected to -theends of said loop aerial; Y f `2. A Ydirectional aerial systemcomprising: a frame aerial, a'vertical aerial coupled thereto, means forinsuring therelectrical symmetry oi said loop including a connectionbetween the electrical Ymidpoint of said loop and ground, a receivercoupled to said aerials, and means including a pair vof crossed linearconductors parallel to' the axis of Vsaid loop and connected to oppositeends of said frame aerial for preventing the horizontal` component of adistorted wave front from setting up out of phase currents in said frameaerial.

WALTER VAN B.- ROBERTS.

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